Blood (low in oxygen and high in
carbon dioxide) enters the right atrium from the
superior and inferior vena cava. The superior vena
cava is the large vein that returns blood to the
heart from the upper half of the body and the
inferior vena cava is the large vein that returns
blood to the heart from the lower half of the
body.

At this point the tricuspid
valve, the valve separating the right atrium from
the right ventricle, is open and blood begins to
fill the right ventricle. The right atrium contracts
to force the rest of the blood into the right
ventricle.

When the right ventricle is
full and begins to contract, the tricuspid valve is
forced shut. (lub)

The right ventricle
contracts and pumps the blood through the pulmonary
artery to the lungs. When the right ventricle
relaxes, the blood in the pulmonary artery forces
the pulmonary valve shut. (dub)

In the lungs, the blood from the body trades its
carbon dioxide for oxygen.

Systemic Side of the Heart:

Now rich in oxygen, the blood
returns from the lungs to the left atrium of the
heart via the pulmonary veins.

The oxygen rich blood fills the
left ventricle. The left atrium contracts to force
the rest of the blood into the left ventricle.

When the left ventricle is full
and begins to contract, the bicuspid valve is forced
shut. (lub)

The left ventricle contracts and
pumps the blood through the aortic valve into the
aorta. The aorta is the main artery supplying
oxygen-rich blood to the body. When the left
ventricle relaxes, the blood in the aorta forces the
aortic valve shut. (dub)

The right and left atria contract at the same
time. The tricuspid and bicuspid valves shut at the
same time creating the "lub" sound. The right and left
ventricles contract at the same time. And the
pulmonary and aortic valves shut at the same time
creating the "dub" sound.